Raman spectroscopy is an optical spectroscopy technique, which measures the inelastic scattering, i.e. Raman scattering of monochromatic light by a material to produce a spectrum characteristic of the material. Raman spectroscopy has been demonstrated to be a powerful non-invasive analytical technology for material characterization and identification.
Conventional Raman spectroscopy generally utilizes a focused laser beam to produce Raman scattering signal from the sample, where the back scattered Raman light is collected and measured with a spectrometer device. Although forward scattered Raman signal can be measured provided it is not completely blocked by the sample, it is not the favored approach in most situations. For transparent samples, the excitation light is much stronger in the forward scattered beam than in the back scattered beam, which makes it harder to be filtered out. For diffusely scattering samples, the Raman signal is attenuated much more strongly in the forward scattered beam than in the back scattered beam, which makes the measurement more time consuming. However, a major drawback of the back scattering geometry is that in diffusely scattering samples, the signal collected is primarily from the surface of the sample, which has shorter scattering path length for both the excitation light and the Raman light than from inside the sample. Thus, if multiple components are distributed in a diffusely scattering sample unevenly, the back scattered geometry will not produce results representative of the sample as a whole.
Transmission Raman measures the forward scattered Raman signal. As the light travels from the front surface to the back of the sample, the scattering path length increases for the excitation light but decreases for the Raman scattered light, such that the difference in contribution from different depths toward the total detected signal is much reduced. For this reason, transmission Raman is the preferred method in content uniformity measurements of pharmaceutical products despite its lower signal throughput. However, due to the complex nature of samples and the unpredictable scattering behavior of the excitation and Raman scattered light, even transmission Raman does not necessarily measure different depths with equal weights. Often, the sample orientation is changed and one or more additional measurements are made, and their average provides a better representation of the sample as a whole. This adds complexity to the measurement.
There thus exists a need for an improved apparatus and method for performing Raman spectroscopy, which not only allows the measurement of a large area of the sample but also enables sub-surface Raman signal collection with two excitation beams penetrating the sample in opposite directions, thereby producing the best possible representation of the sample as a whole without the need of reorienting the sample.